A heat exchanger is used for a vapor-compression refrigerant cycle. Specifically, the heat exchanger is used for an air conditioner in an automotive vehicle. In the air conditioner, the heat exchanger works as a condenser. As shown in FIGS. 8 and 9, a multi-flow type heat exchanger 10 is used in the air conditioner. The heat exchanger 10 includes a pair of headers 3, multiple heat exchange tubes 1, a fin 4 and a side plate 5. The headers 3 are disposed along with a vertical direction of the heat exchanger 10. The heat exchange tubes 1 are disposed in parallel between the headers 3. Both ends of each heat exchange tube 1 are connected to the headers 3, respectively. The fin 4 is disposed between the heat exchange tubes 1. The fin 4 is further disposed outside of the outermost heat exchange tube 1. The side plate 5 is disposed outside of the outermost fin 4.
A separation member 6 is disposed in the header 3 so that the heat exchange tubes 1 are divided into multiple parts P1-P3. Refrigerant is introduced into the heat exchanger 10 from an inlet 7 of the header 3 disposed upper side of the header 3. Then, the refrigerant flows through the parts P1-P3, respectively. While the refrigerant flows through the parts P1-P3, heat is exchanged between the refrigerant in the heat exchange tubes 1 and the outside air outside of the heat exchanger 10 so that the refrigerant is condensed and liquefied. Then, the liquefied refrigerant flows out of the heat exchanger 10 from an outlet 8 of the header 3 disposed under the header 3. The heat exchange tube 1 of the heat exchanger 10 is made of, for example, aluminum. The heat exchange tube 1 is formed by an extrusion method to be flattened. The heat exchange tube 1 includes multiple fluid paths. Each fluid path extends in a longitudinal direction and disposed in parallel in a latitudinal direction, as shown in FIG. 9.
In general, the refrigerant in the air conditioner is, for example, hydro chloro fluoro carbon (i.e., HCFC), or hydro fluoro carbon (i.e., HFC). It is already decided to prohibit using the HCFC refrigerant by year 2020. This is because the HCFC is one of ozone-layer-destroying materials. Further, the HFC refrigerant is one of greenhouse gases. Therefore, the HFC is also strictly limited from discharging to the atmosphere. Thus, alternative materials of chloro fluoro carbon such as the HCFC refrigerant or the HFC refrigerant is required to develop. Specifically, it is required to develop a new technique using the alternative materials.
Recently, carbon dioxide (i.e., CO2) is considered as one of alternative materials. Specifically, the CO2 refrigerant is used in the vapor-compression refrigerant cycle. The CO2 gas is one of natural gasses in nature. Therefore, the CO2 gas does not affects on the global environment substantially compared with the chrolo fluoro carbon.
However, when the CO2 refrigerant is used as the refrigerant in the vapor-compression refrigerant cycle, the CO2 refrigerant has comparatively high pressure in regular use. This is because the refrigerant cycle becomes a super critical refrigerant cycle because of specific thermodynamic properties of the CO2 gas. Therefore, for example, the pressure of the CO2 refrigerant in regular use on a high-pressure side of the refrigerant cycle becomes higher than 10 Mpa. Here, the pressure of the chloro fluoro carbon refrigerant has comparatively low pressure in regular use. The pressure of the chloro fluoro carbon refrigerant is, for example, 3 MPa or 4 MPa. Thus, in a case where the CO2 refrigerant is used as the refrigerant in the refrigerant cycle, it is required to secure the high mechanical strength of the heat exchange tube. Specifically, the heat exchange tube is required to have the withstand pressure three times or more higher than the pressure in regular use on the high-pressure side. That is, the withstand pressure of the heat exchange tube is required to be about 30 MPa or 40 MPa.
A heat exchange tube having high withstand pressure is, for example, disclosed in Japanese Patent No. 3313086 (i.e., Japanese Patent Application Publication No. 2000-356488). A fluid path of the heat exchange tube has a rectangular cross section with a rounding corner. Further, thickness of a sidewall of the heat exchange tube becomes thicker.
However, it is preferred that the fluid path has a perfect circular cross section in view of the withstand pressure of the heat exchange tube. Further, it is difficult to define the withstand pressure on the basis of only a ratio between the thickness of the heat exchange tube and the width of the fluid path. This is because the heat exchange tube can be made of one of various materials having high mechanical strength. Each material has a different mechanical strength. Therefore, it is difficult to estimate the withstand pressure of the heat exchange tube.